One of the more serious problems of electrodialysis stacks is the leakage of liquid from the various compartments. Attempts made include the provision of units of anion-specific membrane, spacer and cation specific membrane in the form of a sealed bag, provided with outlet means. A plurality of such consecutive bags, separated from each other by spacers, and arranged in a suitable housing, with terminal electrode compartments, have been in use for some time. The present invention provides modular units which can be easily assembled, which can be exchanged for new ones if some fault develops, and which overcome the shortcomings of leakage and the like.